In recent years, a secondary battery, which can be charged and discharged, has been widely used as an energy source for wireless mobile devices or an auxiliary power apparatus. In addition, the secondary battery has attracted considerable attention as a power source for electric vehicles (EV), hybrid electric vehicles (HEV), and plug-in hybrid electric vehicles (Plug-in HEV), which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuels.
The secondary battery is used in various kinds of mobile devices that require high output. To this end, a battery pack including a plurality of battery cells connected in series to each other may be used in electric vehicles or hybrid electric vehicles as a power source.
In a middle or large-sized secondary battery module including a plurality of battery cells (unit cells), all or at least some of battery cells constituting the battery module are connected in series to each other in order to provide high output, and the battery cells have the same capacity and voltage standards.
In addition, a plurality of battery packs, each of which includes a plurality of battery cells, is mounted in electric vehicles based on the specifications of the electric vehicles. Furthermore, a charging apparatus and a battery management system are provided to maintain the voltage of the battery cells of the battery packs at an appropriate level while charging or discharging the battery cells.
Although the battery cells have the same capacity and voltage standards, however, the battery cells generally have different voltages due to various factors during the manufacture of the battery cells. If battery cells constituting a battery module have different voltages, the performance of the battery module is lowered.
In addition, the charging apparatus and the battery management system are designed to supply electric current to the battery cells in a lump such that the battery cells are charged with the supplied electric current irrespective of the difference in characteristics between the battery cells. Since the battery cells are charged in a lump irrespective of the difference in capacity between the battery cells or the difference in internal resistance between the battery cells in such a lump charging method, some of the battery cells may be overcharged, or some of the battery cells may not be fully charged.
Furthermore, in a case in which a battery pack including a plurality of battery cells connected in series to each other is used for a long time, characteristic factors of the battery cells may differ from each other as a charging and discharging cycle is increased. As a result, the voltages of the battery cells may be unbalanced. In a case in which the deviation in voltage between the battery cells is excessively increased, the battery cells may be under dangerous situations. For example, the battery cells may explode.
Therefore, there is a high necessity for providing a battery pack balancing method that is capable of stably initializing and controlling battery cells constituting the battery pack in order to fundamentally solving the above problems.